This invention relates to trimming circuits and a method of trimming high accuracy monolithic circuits to improve their yield rate. More particularly, a trimming network and method of trimming thereof to improve the linearity of a digital-to-analog converter (DAC) is described.
With present day integrated circuit technologies, untrimmed wafer yields of high accuracy circuits, for example, DAC's, are too low for profitable production of a low cost part. Because of process tolerances, such circuits must be adjusted in some manner to obtain critical parameters.
The most common approach used today employs active trimming wherein the circuit is functionally connected such that an output is continually monitored while trimming is performed. Trimming is stopped when a predetermined output is reached. This method for adjusting the linearity of a monolithic DAC circuit utilizes laser trimming of the ladder matrix formed of thin film resistors. In addition to being relatively expensive, several problems are associated with active trimming. One such problem is mechanical stress and resulting resistor instability which can be caused by the localized heating of the resistor after it has been trimmed. Furthermore, the light of the laser beam and the heat it generates can also adversely affect sensitive circuitry near the trim and cause the circuit to operate abnormally. Additionally, active trimming is very time consuming, thus expensive, because after each pulse of the laser, transients must be allowed to die out before the output can be measured. Still further, since the trim is performed functionally, noise from the laser equipment, as well as drift of the resistive value and other parameters caused by the heating effect of the laser, can affect the accuracy of the trim.
Another technique in the prior art also employs active trimming. In this technique a thin film resistor is utilized having a plurality of contacts defining incremental resistances therebetween. The contacts are shorted together by links made of gold so that incrementally open-circuiting these links varies the resistance of the resistors. The required trimming of this resistor and iterative measuring of the output thereafter is very time consuming. Moreover, light and heat generated during the functional trimming can still present a problem. Also, the use of gold is very expensive and is not typically compatible with monolithic integrated circuit techniques which generally utilize aluminum.
Thus, a need exists for reducing the cost and other problems associated with trimming high accuracy monolithic circuits such as digital to analog converters.